This invention relates generally to a knock control apparatus and more particularly, to a knock control apparatus equipped with an altitude compensation function capable of appropriately effecting knock control even at a high altitude irrespective of the speed of an engine.
Knock occurring in an engine is accompanied by knocking sound which reduces the engine performance and can cause the drop of the engine output due to the occurrence of reverse torque or the breakdown of the engine due to overheating. Knock is closely related to ignition timing. It is well known that from the engine characteristics, the engine output can be maximized by setting the ignition timing or spark advance angle immediately before knock. Accordingly, if the spark advance angle is reduced so as to avoid the occurrence of knock, the engine output will drop, and the ignition timing must be controlled to the position immediately before the occurrence of knock.
It is also known generally that since the charge in a cylinder becomes lower when driving at higher altitudes than at lower altitudes, the knock occurrence range of the ignition timing advances much more in high altitudes than in the low altitudes. Hence, the engine efficiency can be improved by setting the ignition timing, when driving in high altitudes, at an angle much more advanced than when driving in at low altitudes.
On the other hand, it is already known to carry out altitude compensation of the spark timing by use of a diaphragm or the like in order to control more accurately the ignition timing at high altitudes, as disclosed in Japanese Patent Laid-Open No. 84866/1980, for example. As described above, however, when effecting knock control, the ignition timing must be precisely controlled to the position closest to the knock range by feeding back the knock signal. It has been therefore difficult to apply the mechanical altitude compensation of the prior art described above as such.